In recent years, attempts have been made to domestically raise salmon in rearing farms located in natural waters, usually a mixture of fresh and saline sea water. These consist of large pens which are located at the surface. To date, these pens have not been very successful. Usually, water temperatures are too warm, and the fish are confined at shallower water depths than the ones at which they are comfortable. As a consequence, disease and loss of fish are prevalent.
Biologists who credited the increased salmon runs of 1966 and 1976 on the west coast of North America to improvements in hatchery techniques were largely mistaken. The major reason for the good returns, it has been discovered, was excellent water conditions caused by favourable large scale climatic and oceanographic conditions, noteably upwelling of nutrient rich water. Freshwater rearing conditions, while important, are secondary.
In the winter, natural salt waters stratify, with the colder denser water sinking to the bottom. In the spring and summer, a combination of wind, current and the earth's rotation causes the dense water to come to the surface. The cycle is driven by the typical northerly winds that blow during the summer. These winds tend to push the surface water layer southward. At the same time, the earth's rotation is causing the whole water mass to move from west to east. Because the surface layer is being pushed south, its eastward movement is relatively slower than the water underneath. It is therefore sheared to the west. The underlying water moves in to replace it. This creates a strip of cold water which lies along the west coast of North America in normal years in summer. Recently, the movement northward of the infamous El Nino retarded this upwelling process in two ways. First, by changing weather patterns, the strong northwesterlies were absent during the strong El Nino period. Second, when upwelling occurs in a strong El Nino year, the lower layers of water are not as cold and nutrient-rich as normal.
The nutrients that most frequently control and limit the rate of organic production in the sea are nitrate and phosphate carried in the ambient seawater. Perhaps not coincidentally, these elements are present in seawater in proportions which are very nearly the same as occur in plankton (15-1) (between 0.2 and 1 milligram/liter of water). Phosphorus is a structural component of the nucleic acids and is needed for the transfer of chemical energy within organisms. The cycle in the hydrosphere begins as phosphates, which are leached as dissolved salts or eroded as particles. These find their way to streams and lakes, where they are precipitated or enter living matter. The remainder enters the ocean and are consumed by plankton. As the bodies of planktonic plants and animals sink through the water column, the surface waters necessarily become depleted of phosphorus. Conversely, the deep waters are virtually saturated with calcium phosphate. Additions to the deep ocean pool are balanced by precipitation to the sediment. Upwelling of deep water to shallower waters is the only process which returns a portion of the phosphorus to the surface. Such upwelling generally coincides with rough waters on exposed shores where conditions for raising fish are optimum. Fish rearing pens located in protected bays using surface waters do not take advantage of such optimum conditions. Fish generally are not adapted to develop defensive mechanisms to foreign bodies and thus are unable to survive in unfavourable environments for long periods of time. Surface pens inherently provide an unfavourable environment.
The applicant is aware of the following references which are believed to be more or less pertinent to this invention:
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